1. Field of the Invention and Contract Statement
The invention relates to collar nuts for mechanical seals for pumps.
2. Discussion of Background and Prior Art
Collar nuts for sealing pump shafts are well known in the art. However, collar nuts are reserved usually for adjusting the compression on a series of reeds or packing to form a seal between a machine housing and a reciprocating or rotating shaft. Typically the space between the pump housing and the pump shaft is fitted with packing in a stuffing box and retained with a cylindrical ring threadably fastened to the box. The packing is made of a soft, flexible material. The packing seal prevents lubricant or pumpage from escaping from the space between the pump shaft and the pump housing. In critical or unique applications, mechanical seals are used in place of packing due to their leak-free performance and reduced maintenance requirements.
In some applications, such as where a pump is to operate submerged in a liquid that is especially corrosive or radioactive, mechanical seals have become more elaborate and elastomeric seals have been supplanted by graphite seals. In the form of an O-ring, a graphite seal is very hard, much harder than an elastomeric material such as rubber. Usually the threaded collar nut, or cap nut, is replaced by a cap that has a plurality of bolts for fastening the cap and, as the bolts are sequentially torqued, for providing sufficient force to crush the graphite O-ring against the shaft to form a seal. In order to crush the O-ring properly, a bolt torquing sequence must be carefully and patiently followed.
Fitting and tightening a bolted collar nut can be difficult in a hazardous environment. For example, if the pump is in a radioactive or toxic area, the worker applying the bolted cap must wear protective clothing including a respirator. The length of time taken to torque the bolts in such an environment can become crucial.
If the pump is submerged in corrosive liquids or sludges, it is also important to seal in both directions, against the pumpage entering the pump housing or mechanical seal housing from without and against the lubricant leaving the shaft housing from within.
Finally, in some applications bolt heads themselves present problems. Bolt heads take up critical axial space that may be important since most pumps have limited axial space in which to install mechanical seals. Also, bolt heads, and indeed any surface feature of the exterior of the collar nut, may create turbulence which can adversely affect the integrity of the mechanical seal.